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Subject: Re: [PATCH v4 1/2] tty: hvc: pass DMA capable memory to put_chars()
To:     Arnd Bergmann <arnd@arndb.de>
Cc:     gregkh <gregkh@linuxfoundation.org>,
        Jiri Slaby <jirislaby@kernel.org>, Amit Shah <amit@kernel.org>,
        Omar Sandoval <osandov@fb.com>,
        linuxppc-dev <linuxppc-dev@lists.ozlabs.org>,
        "open list:DRM DRIVER FOR QEMU'S CIRRUS DEVICE" 
        <virtualization@lists.linux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Guo Ren <guoren@kernel.org>
References: <20210806030138.123479-1-xianting.tian@linux.alibaba.com>
 <20210806030138.123479-2-xianting.tian@linux.alibaba.com>
 <CAK8P3a2=BmVv0tvUKaca+LYxuAussAJtAJW9O3fRN2CbV2-9aw@mail.gmail.com>
From:   Xianting TIan <xianting.tian@linux.alibaba.com>
Message-ID: <f18d017b-d6f7-cf87-8859-8d6b50c7c289@linux.alibaba.com>
Date:   Thu, 12 Aug 2021 16:07:58 +0800
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在 2021/8/6 下午10:51, Arnd Bergmann 写道:
> On Fri, Aug 6, 2021 at 5:01 AM Xianting Tian
> <xianting.tian@linux.alibaba.com> wrote:
>> @@ -163,6 +155,13 @@ static void hvc_console_print(struct console *co, const char *b,
>>          if (vtermnos[index] == -1)
>>                  return;
>>
>> +       list_for_each_entry(hp, &hvc_structs, next)
>> +               if (hp->vtermno == vtermnos[index])
>> +                       break;
>> +
>> +       c = hp->c;
>> +
>> +       spin_lock_irqsave(&hp->c_lock, flags);
> The loop looks like it might race against changes to the list. It seems strange
> that the print function has to actually search for the structure here.
>
> It may be better to have yet another array for the buffer pointers next to
> the cons_ops[] and vtermnos[] arrays.
>
>> +/*
>> + * These sizes are most efficient for vio, because they are the
>> + * native transfer size. We could make them selectable in the
>> + * future to better deal with backends that want other buffer sizes.
>> + */
>> +#define N_OUTBUF       16
>> +#define N_INBUF                16
>> +
>> +#define __ALIGNED__ __attribute__((__aligned__(sizeof(long))))
> I think you need a higher alignment for DMA buffers, instead of sizeof(long),
> I would suggest ARCH_DMA_MINALIGN.

As some ARCH(eg, x86, riscv) doesn't define ARCH_DMA_MINALIG, so i think 
it 's better remain the code unchanged,

I will send v5 patch soon.

>
>         Arnd